Abstract: Vitamin C is essential for human health. It is important to estimate the dietary vitamin C intake in the Chinese population to examine the effects of the nutritional transition occurred in recent decades. The present study aimed to estimate the dietary vitamin C intake in Chinese adults by using cross-sectional data from the 2015 China Nutritional Transition Cohort Study and selecting those aged 18–65 years with complete records of sociodemographic characteristics and dietary measurements (n = 11,357). Wilcoxon rank-sum test, Kruskal-Wallis analysis, Chi-squared test, and multiple logistic regression were employed to analyze the daily dietary vitamin C intake on the basis of three-day 24 h dietary recalls and food sources in relation to demographic factors, to evaluate vitamin C intake status using the estimated average requirement cut-off point, and to explore underlying influencing factors. The mean (SD (standard deviation)) and median (interquartile range) levels of the dietary vitamin C intake in adults were 78.1 (54.6) and 65.4 (61.4) mg/day, respectively. Light vegetables, dark vegetables, fruits, and tubers were the top four food sources, contributing a combined 97.3% of total daily dietary vitamin C intake in the study population. The prevalence of risk of insufficient dietary vitamin C intake was 65.1%. Both the distribution of vitamin C intake and the prevalence of risk of insufficient dietary vitamin C intake differed by several demographic factors. Educational level, residence area, geographic location, vegetable consumption, and total energy intake were independent determinants of the risk of insufficient dietary vitamin C intake. In conclusion, dietary vitamin C intake is inadequate in Chinese adult population, and an increase in vitamin C intake should be recommended especially to the population at risk for vitamin C insufficiency. Keywords: vitamin C; dietary intake; food sources; determinants; adults

1. Introduction Vitamin C, also known as ascorbic acid, is a water-soluble vitamin naturally present in some foods, particularly fruits and vegetables. It is used as a food additive and is available as a dietary supplement [1]. Because humans are unable to synthesize it, vitamin C is considered to be an essential dietary micronutrient [1]. The Chinese Dietary Reference Intakes recommend an estimated average requirement (EAR) for vitamin C for adults of 85 mg/day [2]. A regular and adequate intake is required to prevent vitamin C insufficiency. Along with the rapid economic growth and urbanization observed in China over the past few decades, the Chinese population has experienced a nutritional transition characterized by dramatic Nutrients 2018, 10, 320; doi:10.3390/nu10030320

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changes in the dietary patterns, eating, and cooking behaviors [3,4]. Between 1991 and 2011, the consumption of animal source foods increased; over the same time frame, the consumption of coarse grains, legumes, and other healthy foods, e.g., vegetables, declined, and fruit intake remained low [5]. Consistent with these findings, the China Health and Nutrition Survey (CHNS, 1989–2009) showed a decrease in dark vegetable consumption in adults, accompanied by a significantly decreased dietary vitamin C intake, from 117.2 and 111.0 mg/day in 1989 to 85.2 and 83.1 mg/day in 2009 on average for males and females, respectively [6]. Results of the subsequent China National Nutrition and Health Survey (2010–2012) showed that the average dietary vitamin C intake in the Chinese population was 80.1 mg/day, and the mean probability of adequacy of vitamin C was 38.9%, suggesting that dietary vitamin C intake continued to be inadequate in most Chinese [7]. Compared with a reported range from 80 to 230 mg/day of mean vitamin C intake among 27 centers in 10 European countries [8], vitamin C intake in Chinese appeared low [8]. Thus, we must estimate vitamin C intake in the setting of the recent nutritional transition in China before we can recommend interventions to improve dietary vitamin C intake in the Chinese population. Although fruits and vegetables remain the food groups with the highest contribution to total vitamin C intake [7–9], significant disparities in food availability, food choice, and consumption pattern among different ethnic groups worldwide could influence the primary food sources of vitamin C intake from one country to another [10]. In addition, the increasingly westernized lifestyle of China, characterized by the pronounced growth in the consumption of processed foods and beverages and a propensity to eat away from home [5], may lead to changes in the typical pattern of food sources of vitamin C in this country. Besides fruit and vegetable consumption, the role of sociodemographic factors, including income, educational level, and nutritional status [11], and the independent influence of smoking [12] in vitamin C status have been evaluated. In China, previous studies estimated the vitamin C intake stratified by gender, age, residence region, and household income level. However, independent roles of the sociodemographic factors and dietary habits have not been investigated yet, despite evidence that individuals from rural villages or households with low income and members of the aging population have a lower vitamin C intake [6,7,13]. Therefore, the present study aimed to estimate dietary vitamin C intake and vitamin C food sources and evaluate the prevalence of risk of inadequate vitamin C intake in the adult population using newly collected data from the China Nutritional Transition Cohort Study carried out in 2015. To identify independent determinants of the vitamin C intake status, this study further explored potential influencing factors. These findings would be valuable for developing and implementing public health strategies to meet the recommended vitamin C intake and provide guidance for population-specific nutrition education and intervention programs. 2. Materials and Methods 2.1. Study Population Data in the present study were derived from the 2015 China Nutritional Transition Cohort Study conducted by National Institute for Nutrition and Health, Chinese Center for Disease Control and Prevention, which was an amplified survey based on the CHNS [4], an ongoing and longitudinal study established in 1989 by the Chinese Center for Disease Control and Prevention and the University of North Carolina at Chapel Hill. The covered provinces (municipalities) in each round of survey are shown in Figure S1. A multistage, random cluster process was used to draw a sample in each of the provinces (municipalities). The counties and cities in each province were stratified by income (low, middle, and high), and a weighted sampling scheme was used to randomly select four counties and two cities in each province (municipality). Villages and townships within the counties and urban and suburban neighborhoods within the cities were selected randomly. In each community, 20 households were randomly selected, and all household members were interviewed [4]. Compared to the previous surveys, an additional three provinces were included in this wave, according to the

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criteria of substantial variations in geography, economic development, public resources, and health indicators [4]. Subjects aged 18–65 years with complete data of sociodemographic characteristics, anthropometric measures, and dietary surveys were selected to participate in the present study. We excluded women who were pregnant (n = 56) and lactating (n = 108), and subjects with implausible energy intake (5000 kcal/day, n = 63). For participants with dietary consumption values outside the sex-specific limits of dietary consumption distribution (P99 , P: percentile) [1] including dietary vitamin C intake amount (n = 56 of P99 in males, n = 69 of P99 in females), fresh fruit consumption frequency (n = 49 of >P99 in males, n = 71 of P99 in females), and fresh vegetable consumption frequency (n = 50 of P99 in males, n = 62 of P99 in females), we used the corresponding sex-specific P1 and P99 to replace those of P99 , respectively. A total of 11,357 participants were involved in the analysis (Figure S2). Written informed consent was obtained from all subjects before they participated in the study. The study was conducted in accordance with the Declaration of Helsinki, and the protocol was reviewed and approved by the Institutional Review Board of National Institute for Nutrition and Health (No. 2015017, 18 August 2015). 2.2. Measurement of Dietary Intake Detailed information, including amount consumed, cooking method, and eating location, on three meals and snacks for each individual, was obtained through a face-to-face interview of 24 h dietary recalls and recorded using a tablet device (Lenovo ThinkPad Tablet 2, Lenovo, Beijing, China) by trained staff across three consecutive days, including two weekdays and one weekend day [4,14,15]. 2.3. Dietary Estimation and Food Sources of Vitamin C Based on the intake amount of each ingredient of foods consumed during the survey period, the vitamin C content of the reported food items was coded according to the Chinese Food Composition Table [16]. Total vitamin C intake for each person was estimated by summing the vitamin C contribution of each food item and expressed as daily intake (mg/day). The estimated vitamin C intake level was compared with that reported in the Chinese Dietary Reference Intakes [2], in which EAR and recommended nutrient intake (RNI) for vitamin C are 85 and 100 mg/day in adults aged 18–65 years, respectively. The proportion of subjects with a vitamin C intake 500 µg), fruits, and tubers (potatoes), while the remaining thirty food groups were combined into one item because of minor contribution of each food group to vitamin C intake. The breakdown of fruit and vegetable contributors are listed in Table S1. The contribution percentage to total vitamin C intake from various food sources were calculated. 2.4. Dietary Estimation of Total Energy Similarly to the dietary vitamin C estimation described in detail above, the energy content of each reported food item was coded according to the Chinese Food Composition Table on the basis of the individual dietary intake data during the survey period [16]. The total energy intake for each person was estimated by summing the energy contribution from each food item and was expressed as daily intake (kcal/day).

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2.5. Assessment of Fresh Fruit and Vegetable Consumption Frequency The participants were asked to complete a validated semi-quantitative food frequency questionnaire and report whether they consumed sixty-three foods and beverages during the last year and the frequency of that consumption, defined as the average number of times per day, week, month, or year, depending on whether the consumption was usual or unusual. The data on various fruit and vegetable consumption were used for the analysis. For each item, if the participant was a non-consumer, then his/her consumption frequency was set to zero time daily. For consumers, the reported average consumption frequency was converted into a uniform estimate of times/day by dividing times/week by 7, times/month by 30.5, and times/year by 365. Finally, the consumption frequency of each item was summed as that of fruits and vegetables, respectively, which was categorized into three levels (low: >0 and 0 and